1. Field of the Invention
The present invention relates to a display, a method for driving a display, and an electronic apparatus, particularly to a flat-type (flat-panel) display in which pixels, each including an electro-optical element, are arranged on rows and columns (in a matrix), a method for driving the display, and an electronic apparatus including the display.
2. Description of the Related Art
In recent years, in the field of displays for image displaying, an organic electroluminescence (EL) display is being developed and commercialized as one of the flat-type displays obtained by arranging pixels (pixel circuits), each including a light-emitting element, in a matrix. The organic EL display includes organic EL elements as the light-emitting elements of the respective pixels. The organic EL element is a so-called current-driven electro-optical element whose light-emission luminance varies depending on the value of the current flowing through the element, and it is based on a phenomenon that light emission occurs in response to an electric field application to an organic thin film.
The organic EL display has the following features. Specifically, the organic EL display has a low power consumption because the organic EL element can be driven by an application voltage lower than 10 V. Furthermore, because the organic EL element is a self-luminous element, the organic EL display provides a higher image visibility compared with a liquid crystal display, which displays an image by controlling, for each pixel including a liquid crystal cell, the intensity of light from a light source (backlight) by the liquid crystal cell. In addition, the organic EL display does not need to have an illuminating unit such as a backlight, which is necessary for the liquid crystal display, and therefore a reduction in the weight and thickness of the organic EL display can be achieved easily. Moreover, the response speed of the organic EL element is as very high as several microseconds, which causes no image lag in displaying a moving image by the organic EL display.
As the drive system for the organic EL display, a simple- (passive-) matrix system or an active-matrix system can be employed, similarly to the liquid crystal display. However, a display of the simple-matrix system involves the problem that it is difficult to realize a large-size and high-definition display although the configuration thereof is simple. For this reason, in recent years, a display of the active-matrix system is being actively developed in which the current flowing through an electro-optical element is controlled by an active element, such as an insulated gate field effect transistor (typically, a thin film transistor (TFT)), provided in the same pixel circuit as that including this electro-optical element.
It is generally known that the I-V characteristic (current-voltage characteristic) of an organic EL element deteriorates as the time elapses (so-called age deterioration). In a pixel circuit including a N-channel TFT as a transistor for driving an organic EL element by current (hereinafter, referred to as a “drive transistor”), the organic EL element is connected to the source side of the drive transistor. Therefore, the age deterioration of the I-V characteristic of the organic EL element leads to a change in the gate-source voltage Vgs of the drive transistor, which results in a change in the light-emission luminance of the organic EL element.
A more specific description about this point will be given below. The source potential of the drive transistor is determined depending on the operating point of the drive transistor and the organic EL element. The deterioration of the I-V characteristic of the organic EL element varies the operating point of the drive transistor and the organic EL element. Therefore, even when the same voltage is applied to the gate of the drive transistor, the source potential of the drive transistor varies. This varies the source-gate voltage Vgs of the drive transistor, which changes the value of the current flowing through the drive transistor. As a result, the value of the current flowing through the organic EL element also changes, which varies the light-emission luminance of the organic EL element.
Furthermore, a pixel circuit employing a poly-silicon TFT involves, in addition to the age deterioration of the I-V characteristic of the organic EL element, changes over time in the threshold voltage Vth of the drive transistor and the mobility μ in the semiconductor thin film serving as the channel of the drive transistor (hereinafter, referred to as “the mobility of the drive transistor”), and a difference in the threshold voltage Vth and the mobility μ from pixel to pixel due to variation in the manufacturing process (variation in transistor characteristics among the respective drive transistors).
If the threshold voltage Vth and mobility μ of the drive transistor are different from pixel to pixel, a variation in the value of the current flowing through the drive transistor arises on a pixel-by-pixel basis. Therefore, even when the same voltage is applied to the gate of the drive transistor, the variation in the light-emission luminance of the organic EL element among the pixels arises, which results in a lowered uniformity of the screen.
To address this problem, there has been proposed a configuration aiming to allow the light-emission luminance of the organic EL element to be kept constant without being affected by the age deterioration of the I-V characteristic of the organic EL element and changes over time in the threshold voltage Vth and mobility μ of the drive transistor. Specifically, in this configuration, each of the pixel circuits is provided with a compensation function against change in the characteristic of the organic EL element, and correction functions for correction against variation in the threshold voltage Vth of the drive transistor (hereinafter, referred to as “threshold correction”) and correction against variation in the mobility μ of the drive transistor (hereinafter, referred to as “mobility correction”) (refer to e.g. Japanese Patent Laid-Open No. 2006-133542 (hereinafter referred to as Patent Document 1)).
By thus providing each pixel circuit with the compensation function against change in the characteristic of the organic EL element and the correction functions against variation in the threshold voltage Vth and mobility μ of the drive transistor, the light-emission luminance of the organic EL element can be kept constant without being affected by the age deterioration of the I-V characteristic of the organic EL element and changes over time in the threshold voltage Vth and mobility μ of the drive transistor.
In an organic EL display employing the configuration in which each pixel circuit is provided with the correction functions for threshold correction and mobility correction as described above, the following four kinds of operation are cyclically carried out for each pixel row: threshold correction preparation in which the gate potential Vg and source potential Vs of the drive transistor are fixed at the respective predetermined potentials; threshold correction in which the source potential Vs of the drive transistor is sufficiently raised to fix the gate-source voltage Vgs of the drive transistor at the threshold voltage Vth thereof; signal writing in which the signal voltage Vsig of a video signal dependent upon luminance information is written in the pixel; and mobility correction in which correction relating to the mobility μ is carried out (details of the respective operations will be described later.
In the case of carrying out these four operations in the period of 1 H (H denotes the horizontal scanning period/horizontal synchronizing cycle) for each pixel row, there is a problem that it is difficult to ensure, as the threshold correction period and the mobility correction period, sufficiently-long times to carry out the respective correction operations surely. In particular, the number of pixels tends to be increasing year by year in linkage with the enhancement in the display definition, and the period of 1 H is becoming shorter along with the increase in the number of pixels. Therefore, currently, it is becoming difficult to ensure sufficiently-long times as the threshold correction period and the mobility correction period.
Not only in the organic EL display provided with both the correction functions for threshold correction and mobility correction but also in an organic EL display provided with only the threshold correction function, the time that can be ensured as the threshold correction period becomes shorter due to the shortening of the period of 1 H.
If sufficient time may not be ensured as the correction period for threshold correction or the respective correction periods for threshold correction and mobility correction, the threshold correction operation or the respective correction operations of threshold correction and mobility correction may not be surely carried out. This results in a failure in sufficient suppression of the variation in the current flowing through the drive transistor from pixel to pixel. Therefore, as described above, even when the same voltage is applied to the gate of the drive transistor, the variation in the light-emission luminance of the organic EL element among the pixels arises, which lowers the uniformity of the screen.